That's the opposite of what's true.Multiple independent historical sources, both Christian and non-Christian, attest to the crucifixion.
This is evidence. It is the same kind of evidence that we have for most ancient history.
All outside sources are considered later additions by the church or simply mentioning Christians who follow he gospels.
The gospels are not historical and read exactly as myth and are all copied from of Mark.
The Paul that is authentic mentions only scripture and revelation, no life of Jesus.
Like historian Richard Carrier says the evidence "sucks".